Multi-grade vs. single-grade oils: the debate continues

n recent months I have had several conversations with the people at Precision Engines in Everett, Wash., about the use of multi-grade oils in large radial engines.

n recent months I have had several conversations with the people at Precision Engines in Everett, Wash., about the use of multi-grade oils in large radial engines.

Multi-grade oils are the preferred choice for automotive and heavy duty truck applications. In aviation they seem to work well in flat engines and smaller radial engines, especially in winter.

However, Precision Engines has noted an increased failure rate and/or wear level in master rod bearings in radial engines larger than 1,800 cubic inches displacement when multi-grade oils are used. This failure/wear rate is primarily in non-conventional usage, such as war birds and other non-commercial uses.

Although the results are not statistically significant at the 95% confidence level, they seem to be a strong indication of a problem.

The problem is the master rod bearing design in radial engines. In a flat or V engine, each connecting rod bearing transmits the power from just that one cylinder. This means that in a 400 hp flat engine each rod bearing must handle less than 70 hp per power cycle ? and the bearing is completely unloaded during the power cycle. When an engine completes the exhaust cycle and starts the intake cycle, the bearing goes from a push to a pull mode and the bearing cavity is completely filled with oil for the start of the compression and power cycle. The mechanical movement of the bearing and crank journal help force oil and maintain a gap between the two parts as the journal revolves.

By comparison, in a radial engine the master rod bearing must transmit the entire load for that row of cylinders. Thus, in a large radial engine, the master rod bearing may have to transmit up to 1,400 hp. In addition, during the operation of a nine-cylinder radial engine, there is a power pulse every 80?. This means that the bearing is never unloaded. Therefore, the only two factors that determine the oil film thickness between the bearing and the crankshaft are the pressure in the oil system and the leakage rate out of the bearing cavity.

Here lies the problem with multi-grade oils. Because of their improved flow characteristics, they tend to leak better than single-grade oils under the same conditions. I am pretty sure that every radial engine operator and some flat engine operators have noted an increase leakage rate when they switched from single-grade oil to a multi-grade oil. This improved flow rate is the big advantage of multi-grade oils on a cold engine start up. But in a large radial engine master rod bearing, this improved flow rate means more leakage out of the bearing cavity and a reduced oil film thickness during the power pulse.

In normal operation this reduced film thickness is not a problem. However, if the engine is subject to sharp power changes ? or worse, actual reverse loading?? the master rod bearing may encounter actual metal to metal contact with the crank shaft surface. When this occurs, the bearing material is hammered out a small amount. With each reoccurrence, the bearing clearance is increased, which reduces the oil film thickness even further. This process continues until bearing failure can occur.

The effect of multi-grade oils on continuously loaded bearings is not unique to radial engines. For example, the connecting rod bearings in two-cycle diesel engines also are continuously loaded. Many operators have observed reduced wear in these engines when single-grade engine oils are used, especially in larger engines.

The bottom line is, if you operate a large radial engine of over 1,800 cubic inch displacement, I recommend that you use a single-grade 120 engine oil approved for your engine, especially in warm temperatures. An approved grade 100 oil can be used in colder temperatures as per the appropriate service documents.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at

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